Natural gas hydrate (NCH) that is composed mostly of methane exists under the seabed not greater than 500 m deep surrounding the continent and in the frozen areas such as Siberia, Canada and Alaska. The NGH is water solid substance or clathrate hydrate of which primary components are gas molecules of methane and others and water molecules, this water solid substance or clathrate hydrate is stable at low temperature under high pressure. The NGH draws attention as clean energy with low emission of carbon dioxide and air pollutant.
In general, natural gas is liquefied, and then stored to be used as energy. The liquefied natural gas is produced and stored at extremely-low temperature of −162° C. On the contrary, the natural gas hydrate has the advantage of exhibiting stable properties without decomposition and being handled as a solid at temperature of −20° C., and so on. Since the natural gas hydrate has such properties, a natural gas hydrate method (NGH method) involving formation, transportation, storage and regasification of natural gas is expected as means of effective use of gas resources in small- and medium-scale gas fields all over the world which have not been developed due to profitability reasons, or in such a case that a small lot is transported a short distance from a large-scale gas field.
In the NGH method, NGH is formed in a format suitable for transportation and storage at a NGH shipping site such as a small- or medium-scale gas field, and the NGH is transported to a NGH receiving site as designated by a vessel, vehicle or the like. At the NGH receiving site, the transported NGH is stored and used by gasification in a gasification apparatus as energy source when needed. FIG. 5 is a schematic view for explaining an example of a structure of the plant which is used at the NGH shipping site for forming a gas hydrate. A mined raw material gas G is hydrated by being mixed fully with water W in a generator 1 that is a high-pressure reaction vessel, thereby a low-density gas hydrate (GH) slurry is generated. The generated GH slurry is fed in a dewaterer 3 by a feed pump 2, then dewatered, and thereby a high-density GH slurry is obtained. At that time, the GH slurry is fed into a bottom of the dewaterer 3. The fed GH slurry goes upward in the dewaterer 3. The GH slurry is dewatered while going upward at a draining portion (a portion provided with micropores, slits or the like for separating hydrate particles from water) which is provided on a mid-position its way up in the dewaterer 3, and is taken out of an upper end portion of the dewaterer 3. The gas hydrate taken out is in the form of GH cake. The GH cake is fed in a pellet molding apparatus 4 for pelletization, and molded into pellets having a size suitable for transportation, storage or the like. Subsequently, the GH pellet is cooled by a cooler 5 to such a temperature at which the GH pellet does not decompose under ambient pressure, and then fed in a depressurizer 6. The process steps for the gas hydrate from the generator 1 before the cooler 5 is performed at room temperature under high pressure which is the condition for forming the gas hydrate. Then, the gas hydrate is processed to such a temperature that the gas hydrate does not decompose under ambient pressure in the cooler 5 and the depressurizer 6. After that, the molded GH pellets are fed to and stored in a storage tank.
By the way, the applicant of the present application proposes the method and the apparatus for producing gas hydrate pellets, allowing for production of gas hydrate pellets excellent in storability at low cost (refer to Patent Document 1). According to the method for producing gas hydrate pellets, a gas hydrate is dewatered by compression and molding means under the condition of forming the gas hydrate so that a gas hydrate is generated with raw material gas between the gas hydrate particles and water, and thereby a gas hydrate pellet is produced. And, as the compression and molding means, used is a briquetting machine comprising a pair of rollers rotating in opposite directions and each having an outer peripheral surface provided with a plurality of molds for pellets.
Further, the applicant of the present application proposed an apparatus for molding gas hydrate pellets to improve the efficiency in molding the GH pellets by performing a dewatering process and a molding process of the GH pellets by using a single device in a gas hydrate formation plant (refer to Patent Document 2). In the apparatus for molding gas hydrate pellets according to Patent Document 2, a compression plunger is arranged in an inner cylinder of a compression chamber, water is squeezed out from GH slurry which is fed into the inner cylinder by advancing the compression plunger, and the water is drained through a screening portion provided in a part of the inner cylinder. After the water is squeezed out, a gate valve is opened, the GH pellet P is pushed and moved into a cooling chamber through the gate valve by further advancing the plunger. Then, the gate valve, is closed, the cooling chamber is cooled, and following GH slurry is fed subsequent to retreat of the compression plunger.